This invention relates to optoelectronics, and in particular to an optical device which includes a tap for monitoring the light from the device.
Optical fiber networks have become increasingly important in modern communications where large information handling capacity is needed. Of particular interest are Dense Wavelength Division Multiplexing (DWDM) systems where several wavelengths of light from a source are carried by a single fiber. This increase in system complexity has been accompanied by an increase in the complexity and level of integration of the optical components from which they are constructed. The optical sources for such a DWDM system preferably consists of a wavelength tunable semiconductor laser, such as Distributed Bragg Reflector (DBR) laser, integrated with a semiconductor optical amplifier (SOA) to boost the output power, a high speed electroabsorption (EA) modulator, as well as provisions for monitoring and controlling the optical power and wavelength of the output signal. The electrical outputs of these monitors are typically used as part of one or more feedback loops to control the wavelength and output power of the transmitter.
For typical discrete single-section semiconductor lasers, such as Distributed Feedback (DFB) lasers, the unused light emitted from the rear facet of the laser can be directed into an arrangement of wavelength discriminators and photodetectors in order to perform the monitoring functions. This is possible because the fiber-coupled output light is directly proportional to the rear facet light. If additional optical elements such as an SOA are integrated after the laser, then the output power of the integrated laser plus SOA is not necessarily proportional to the back facet light, so this method can no longer be used. A typical approach in these cases is to utilize an optical splitter to tap off a small portion of the output light, and to direct the tapped portion to a photodetector which absorbs all of the light and converts the optical signal to an electrical signal used in a feedback loop. Positioning the splitter outside the chip is not desirable, since it adds expensive components and assembly costs to the transmitter.
It has been recognized that it would be more economical to integrate the splitter and photodetector functions on the same semiconductor chip as the other optical elements. One such device includes a branching waveguide to supply a portion of the light from the amplifier to the photodetector. (See U.S. Pat. No. 5,134,671 issued to Koren, et al.) Another device employs an integrated photodetector with an absorption layer placed with respect to the waveguide propogating light from the amplifier so that it absorbs only a portion of the light from the amplifier. (See U.S. Pat. No. 5,029,297 issued to Halemane et al.) While such devices are effective, it is believed that they add undue complexity to the device structure, either in requiring a branched waveguide or additional layers in the photodetector structure.
It is desirable, therefore to provide an integrated, active semiconductor light emitting device and photodetector which is economical and easily fabricated.
The invention in accordance with one aspect is a semiconductor device including an active device, a waveguide layer in which light is propogated through the active device, and an integral photodetector formed in the device. A light absorbing layer of the photodetector comprises a portion of the waveguide layer which is made to absorb a portion of the light while transmitting the remainder of the light.
In accordance with another aspect, the invention is a method for forming a semiconductor device wherein a semiconductor waveguide layer adapted for light propogation is formed over a substrate, a portion of the layer is made partially absorbing of the light, an active optical device is formed including the waveguide, and a photodetector device is formed including the partially absorbing portion of the waveguide.